Nickel-based catalyst systems including a nickel-containing compound, an organoaluminum compound, and a fluorine-containing compound may be employed for polymerizing 1,3-butadiene to form cis-1,4-polybutadiene. The fluorine-containing compounds may include boron trifluoride and complexes of boron trifluoride with monohydric alcohols, phenols, water, mineral acids, ketones, esters, ethers, and nitriles. It has been suggested in the prior art that blends of alkyl aluminum compounds, such as those containing C2 and C8-C12 alkyl groups can be employed to produce high cis-1,4-polybutadiene with reduced microgel. It has also been suggested that polymers with reduced cold flow can be achieved by adding the catalyst system in the presence of a small amount of monomer.
Because it may be desirable to produce cis-1,4-polybutadiene with lower molecular weight, efforts have been made to control the molecular weight of the resultant polymers produced with nickel-based catalysts. For example, the polymerizations may be conducted in the presence of non-conjugated diolefins or olefins such as 1-butene, isobutylene, cis and trans-2-butene, and allene. In other instances, molecular weight reduction has been achieved by conducting the polymerization in the presence of halogenated phenols or para-styrenated diphenylamine.
Also, the molecular weight distribution of nickel-synthesized polybutadienes has been controlled by conducting the polymerization in the presence of halogenated aldehydes and/or quinone compounds. In other instances, molecular weight distribution has been controlled by conducting the polymerization in the presence of carboxylic acids.
Still further, the degree of branching of the nickel-synthesized cis-1,4-polybutadienes has been controlled by conducting the polymerization in the presence of dialkyl zinc compounds.
While several advancements have been made in the production of cis-1,4-polybutadiene produced with nickel-based catalyst systems, the fact that these nickel-based systems are advantageously employed in many commercial applications, there remains a need to continue improvements. For example, there remains a desire to control gelation, control branching, control molecular weight, and improve yield and reaction times.